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Operations and Significant Accounting Policies - Other Income (Expense) - Other (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Other Income (Expense) - Other [Line Items]      
Total Other Income (Expense) - Other $ 8.7 $ 14.7 $ 18.7
Pension and Other Postretirement Benefit Plans Non-Service Credit [Member]      
Other Income (Expense) - Other [Line Items]      
Total Other Income (Expense) - Other [1] 6.1 8.6 7.7
Interest and Investment Earnings [Member]      
Other Income (Expense) - Other [Line Items]      
Total Other Income (Expense) - Other 2.3 1.6 4.4
AFUDC - Equity [Member]      
Other Income (Expense) - Other [Line Items]      
Total Other Income (Expense) - Other 2.6 1.9 2.3
Gain on Land Sales [Member]      
Other Income (Expense) - Other [Line Items]      
Total Other Income (Expense) - Other 0.1 0.4 2.1
Other Income (Expense) - Other [Member]      
Other Income (Expense) - Other [Line Items]      
Total Other Income (Expense) - Other $ (2.4) $ 2.2 $ 2.2
[1] These are components of net periodic pension and other postretirement benefit cost other than service cost. (See Note 11. Pension and Other Postretirement Benefit Plans.